Dynamo hub wattage options

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Mr_average
Posts: 29
Joined: 6 Sep 2013, 7:15am

Dynamo hub wattage options

Post by Mr_average »

I’m after a dynamo hub to replace a recently deceased bottle generator on the commuter and was surprised to find models listed as ‘1.5w’:

http://www.rosebikes.co.uk/article/shimano-deore-xt-dh-t785-1d-disc-hub-dynamo/aid:742795

It’s described as ‘especially designed for use with LED lights’ (which is obviously what I’m using) so would this be the best model for me? I currently have an Axa Echo 30 front light and a B&M Toplight rack mounted LED rear. How will the light output I get differ from my previous 6v 3W B&M bottle, or the alternative 6v 3W hub generators that are still listed:

http://www.rosebikes.co.uk/article/shimano-hub-dynamo-deore-xt-dh-t785/aid:515917

Thanks
Brucey
Posts: 44522
Joined: 4 Jan 2012, 6:25pm

Re: Dynamo hub wattage options

Post by Brucey »

well, the lights could be about half as bright....?

They will still be bright enough to be seen round town though.

The 1.5W generators are comparable with the less expensive 3W generators, but they are not half the weight and they even have comparable parasitic drag (i.e. when the lights are not on). A good (i.e. more expensive) 3W generator is probably still a better choice. For example a DH-3N80 has lower lights off drag than most of the 1.5W generators, IIRC.

To confuse you even further you can also get 2.4W units (as meant for the Dutch). At the right price these can make a good buy; they are almost as good as a 3W unit but there is a bit less drag than a comparable 3W unit.

Right now SJS are doing 2.4W and 1.5W units very cheaply. Annoyingly they don't come with the connector plug though!

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
Mr_average
Posts: 29
Joined: 6 Sep 2013, 7:15am

Re: Dynamo hub wattage options

Post by Mr_average »

Thanks Brucey - the half as bright bit was my first reaction too but I assumed I must be missing something when it said they were for LED lights. I'll check those SJSC offers but it sounds like a 3W one is the way to go.
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andrew_s
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Joined: 7 Jan 2007, 9:29pm
Location: Gloucestershire

Re: Dynamo hub wattage options

Post by andrew_s »

They say they are for LED lights because in Germany it would be illegal to use one with a filament bulb, on account of not being bright enough.
Mr_average
Posts: 29
Joined: 6 Sep 2013, 7:15am

Re: Dynamo hub wattage options

Post by Mr_average »

That makes sense - so where I interpreted 'especially designed for use with LED lights' as meaning 'the best hub for LED lights', what it meant was 'only useable with LED lights'. I'm sure it was unambiguous in the original German or Japanese.
edocaster
Posts: 475
Joined: 10 Apr 2013, 10:43pm

Re: Dynamo hub wattage options

Post by edocaster »

I really can't see the point of these 1.5W dynamos. The small difference in weight/drag compared to a 3W dynamo probably can't be felt blindfolded, and wouldn't really attract weight weenies enough. And you still need to build a whole wheel around these, which puts them at a disadvantage to contactless rim dynamos under development.
Brucey
Posts: 44522
Joined: 4 Jan 2012, 6:25pm

Re: Dynamo hub wattage options

Post by Brucey »

well I'm not sure they are entirely well thought-out either but if the hub is cheap enough it adds only ~200g to the weight of a normal hub and even a bad 1.5W dynamo only drags 3-4W or so. If this provides the facility to have reliable urban lights ( for another 100g say) then you stand a fighting chance of building a lightweight city bike...?

I'd probably add another 150g and have a full-fat 3W generator myself....

In comparison adding puncture resistant tyres will cost about the same, drag about 10W, and add a lot more than 200g (more like 400g).

Horses for courses?

cheers
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~~~~~~~~~~~~~~~~~~~~~~Brucey~~~~~~~~~~~~~~~~~~~~~~~~
MikeF
Posts: 4339
Joined: 11 Nov 2012, 9:24am
Location: On the borders of the four South East Counties

Re: Dynamo hub wattage options

Post by MikeF »

Some of the cheaper 1.5W are heavier than the dearer 3W ones!
"It takes a genius to spot the obvious" - my old physics master.
I don't peddle bikes.
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